In a typical proton therapy system used for tumor radiation treatments for example, a proton beam is produced in a cyclotron or a synchrotron in a specific level of energy that can be adjusted to a prescribed energy level by virtue of energy selection then provided to a treatment station via a beam transportation system. Such a therapy system includes a particle accelerator, such as a cyclotron or a synchrotron, for providing the particle beam at a specific energy level. The beam transport system can tune and deliver the particle beam to a radiation station. At the end of the beam transport system, a rotational gantry associated with a radiation nozzle delivers the beam onto an irradiation object, e.g. a tumor of a patient, in a fixed position supported by the irradiation station during operation. Similar systems can be used for other heavy particle radiation treatment, such as neutron, He or C ion beam.
Typically a beam output from an accelerator has a fixed energy, e.g. 250 MeV. Depending on the diagnosis of a patient's condition, for example the depth of a tumor to be treated, different patients are prescribed with different depth doses of radiation. An energy selection system (ESS) is usually used to tune the fixed energy to the prescribed energy, e.g. 170 MeV. Conventionally, an ESS comprises an energy degrader for attenuating the beam energy roughly, followed by a set of energy selection dipole magnets dedicated for fine energy selection by filtering the undesired traverse emittances, momentum spread and energy spread resulted from the energy degrader. The transport system also includes a plurality of other magnets for beam focusing and steering purposes.
Due to the high cost for purchasing and maintaining such a radiation system, a medical facility usually uses one accelerator for a plurality of treatment stations so the high expenditure for the accelerator facilities is distributed. FIG. 1 illustrates a configuration of a medical facility that accommodates a proton radiation system 100 providing proton beams for multiple treatment stations in accordance with the prior art. The system 100 comprises a single stationary cyclotron 101 located in a dedicated room 110, a carbon wedge energy degrader 102 disposed in a vacuum component of the beam line, a gantry 121 and 122 for each treatment room 131 and 132, and an ESS, several sets of quadrupole magnets for focusing the beam, e.g. 104, and a plurality sets of bending magnets that directs the proton beams from the cyclotron to respective treatment rooms, e.g. 131 and 132. As shown, the ESS of this system is composed of a carbon wedge degrader 102, and two dipole magnets 105 and 106 with an energy slit (not explicitly shown) sitting in between. The dipole magnets 105 and 106 are located proximate to the accelerator 101 and dedicated for selectively passing the particles with the targeted energy.
In order to supply the particle beams to different rooms located in various places relative to the accelerator room 110, the system 100 is equipped with long beam lines, e.g. 111 and 112, along different paths in which dipole magnets are used to change beam directions. For example, dipoles 107 and 108 are used to redirect the particle beam into the room 110. The dipole 141 bends the beam by 45° at the entrance of the gantry 121. Another dipole 142 bends the beam by 135° and toward the isocenter. Collectively, the two dipoles 141 and 142 in the gantry bends the beam by 90° from the beam line 111.
Although using a multi-station single-cyclotron system is effective to distribute the cost for large medical facilities, the overall cost for such a multi-gantry system may be prohibitively high for smaller facilities that may only need one treatment station. Also, some multi-station systems do not support simultaneous treatment in multiple stations. This contribute to further disadvantage that a delay at one treatment station can cause delay at the other station. Among the costly factors in the conventional proton radiation system, the dipole magnets consume significant expenditure associate with manufacture, installation, control, maintenance, and space that is limited and valuable in the medical facility.
Moreover, connecting to the stationary cyclotron and the rotating gantry, the beam line pipe comprises a rotating portion that can rotate along with the gantry and a stationary or non-rotating portion leading to the cyclotron, both portions being maintained under continuous low pressure (vacuum) typically in the 10E-05 mbar range. Conventionally, a rotating vacuum seal is used at the beam line connection between the stationary part of the beam line and the rotating part of the beam line to keep the pipe sealed from outside air during rotation.